1. Field of the Invention
The present invention relates generally to gas turbine engine, and more specifically for a multiple piece turbine rotor blade.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an industrial gas turbine (IGT) engine, a hot gas stream is passed through a turbine to produce mechanical energy. It is well known that the efficiency of the turbine, and therefore of the engine, can be increased by passing a higher temperature gas stream through the turbine. This is known as the turbine inlet temperature. The highest turbine inlet temperature is limited to the material properties of the turbine, especially the first stage stator vanes and rotor blades, since these airfoils are exposed to the highest temperature gas stream.
Higher turbine inlet temperatures can be can be obtained with a combination of improved material properties that will allow higher temperature and improved airfoil cooling. Prior art turbine rotor blades and made from nickel super alloys produced by the investment casting process. It has been proposed in the past to form the blades from high temperature resistant materials such as tungsten or molybdenum or columbium. These materials have melting temperature so high that they cannot be cast or machined using investment casting processes.
The applicant has proposed to form a turbine blade or stator vane from one of these exotic high temperature resistant materials in which the blade is formed with multiple pieces. one such embodiment is the spar and shell configuration in which a shell having an airfoil shape with a leading edge and a trailing edge, and a pressure side wall and a suction side wall, is formed from one of these exotic high temperature resistant materials using a wire EDM process for cutting the shell into its desired shape from a block of these materials. The shell is then secured to the spar and tip cap by clamping the shell between the tip cap and the platform of the blade. In order to use this spar and shell configuration, a separate tip cap from the spar is required. However, because the blade is a turbine rotor blade, the tip cap is exposed to high stress levels due to the centrifugal force developed from blade rotation.
with a separate tip cap secured to the top end of the spar, the applicant has discovered that very high stresses occur in the connection between the tip cap and the spar because all of the weight from the shell must be passed into the tip cap and then through the connections to the spar. This design—with a separate tip cap and the centrifugal loads from the shell being passed into the tip cap—produces very high stress levels in the tip cap to spar connection structure. A typical tip cap to spar connection that the applicant has used in a dovetail slot and groove connection.